This invention relates to a polymer electrolyte fuel cells, particularly to an electrodes therefor and a methods and apparatuses of manufacturing the same, and more specifically to an improvement of a catalyst layer of the electrodes.
In a fuel cell using a polymer electrolyte, a fuel gas containing hydrogen and an oxidant gas containing oxygen, such as air, are electrochemically reacted so as to generate electricity and heat simultaneously. This fuel cell basically comprises a polymer electrolyte membrane for selectively transporting hydrogen ions and a pair of electrodes placed on both surfaces of the electrolyte membrane. The electrode is constituted by a catalyst layer having, as a main ingredient, a carbon powder carrying a platinum group metal catalyst, and by a gas diffusion layer made of, e.g., a carbon paper having been subjected to a water repellency treatment, formed on an outer surface of this catalyst layer and having both gas permeability and electronic conductivity.
In order to prevent supplied gases from leaking to the outside, and to prevent an oxidant gas and a fuel gas from intermingling with each other, a gas sealing material and a gasket are placed at a peripheral portion of the electrode to sandwich the polymer electrolyte membrane. This sealing material and the gasket are preliminarily assembled integrally with the electrode and the polymer electrolyte membrane. This assembly is called an MEA (electrolyte membrane-electrode assembly). Outside the MEA are placed electrically conductive separator plates for mechanically fixing the MEA and for electrically connecting neighboring MEAs in series with each other. At portions of the separator plates, which portions are to contact the MEAs, gas flow channels are formed for supplying reactive gases to surfaces of the electrodes and for carrying away generated gases and excessive gases. The gas flow channels can be provided separately from the separator plates, but it is conventional to provide grooves, as gas flow channels, on the surfaces of the separator plate.
For a catalyst layer in a polymer electrolyte fuel cell, a thin sheet made by forming a mixture of a fine carbon powder, carrying a platinum group metal catalyst, with a polymer electrolyte is generally used. Usually, it is conventional to make this catalyst layer by: mixing a fine carbon powder carrying a catalyst with an alcohol solvent, such as ethanol, having a polymer electrolyte dissolved or dispersed therein; adding to such mixture an organic solvent, such as isopropyl alcohol or butyl alcohol, having a comparatively high boiling point to make an ink; and forming the layer by a screen printing process, a spray coating process, a doctor blade process, a roll coating process, or the like. The concentrations of commercially available polymer electrolyte solutions or dispersions are at most about 10%, which are not high concentrations. Accordingly, in order to mix a comparatively large amount of polymer electrolyte with a fine carbon powder carrying a catalyst, a large amount of polymer solution or dispersion has to be mixed with the fine carbon powder carrying the catalyst. Therefore, in some cases, an ink made by mixing a polymer electrolyte solution or dispersion with a fine carbon powder carrying a catalyst decreases too much in its viscosity to obtain a desired high viscosity ink.
Thus, there is another process, in which a solvent of an ink is evaporated, so as to obtain a high viscosity ink. However, according to such process, it is difficult to make highly reproducible inks, depending on ink lots. Therefore, there is still another method used for preparing an ink which is: to preliminarily evaporate and solidify a polymer electrolyte solution or dispersion; to dissolve or disperse it in an organic solvent, such as isopropyl alcohol or butyl alcohol, having a comparatively high boiling point, thereby to make a solution containing a desired concentration of polymer electrolyte; and to mix therewith a fine carbon powder carrying a catalyst. At any rate, it has been conventional to use, as a solvent for an ink, an alcohol solvent such as propyl alcohol or butyl alcohol, having a comparatively high boiling point in comparison with solvents such as methanol and ethanol having low boiling points, in order to avoid a change of the ink concentration within a short time.
As described above, when an ink to form a catalyst layer is prepared by adding a further solvent to a liquid mixture of a solution or dispersion of a polymer electrolyte with a fine carbon powder carrying a catalyst, the kinds of solvents to be added have much influence on the material properties of the polymer electrolyte to be mixed with the carbon powder, and consequently much influence on cell performance. Generally, a higher performance electrode can be obtained as comparatively thinner layers of polymer electrolyte are more uniformly adhered to surfaces of a fine carbon powder carrying a catalyst. If, with a solution or dispersion of polymer electrolyte, an organic solvent or water or the like having a polarity extremely different from that of the above is mixed, the polymer electrolyte having been dissolved or finely dispersed therein is agglomerated and separated. If an ink made of such solution is used, comparatively big agglomerates of the polymer electrolyte are adhered to the surfaces of the fine carbon powder carrying the catalyst, so that the electrode performance decreases. Thus, it has been a general practice to use, for preparing inks, alcohol solvents having properties comparatively closer to those of the solution or dispersion of the polymer electrolyte, or organic solvents having comparatively strong polarities, such as butyl acetate, particularly alcohol solvents having comparatively high boiling points in view of the slowness in their evaporation. Further, an alcohol solvent having a comparatively high boiling point has generally been used for the above reason also in the case when an ink is made by: preliminarily evaporating and solidifying a solution or dispersion of polymer electrolyte; dissolving or dispersing the same in an organic solvent, such as isopropyl alcohol or butyl alcohol, having a comparatively high boiling point, thereby making a solution or dispersion of polymer electrolyte having a desired solvent and a desired concentration; and mixing therewith a fine carbon powder carrying a catalyst.
Further, in order to uniformly adhere electrolyte layers to surfaces of a fine carbon powder carrying a catalyst, it has been an indispensable step, for the above reason, to mix an organic solvent containing the polymer electrolyte with a fine carbon powder carrying the catalyst.
On the other hand, a gas diffusion layer is usually constructed of a porous carbon layer, such as a carbon non-woven fabric, having been subjected to water repellency treatment, and is provided, in some cases, with a water repellent carbon layer on a surface thereof interfacing with a catalyst layer, for the purpose of keeping the catalyst layer or the polymer electrolyte membrane in a wet condition. The water repellent carbon layer is usually made as follows. First, fine carbon particles and a dispersion of fine fluorocarbon resin particles containing a surfactant are mixed with each other, and are processed, e.g., by drying or filtering, thereby to obtain a mixture of the fine carbon particles with the fine fluorocarbon resin particles. This mixture is converted to an ink by using water or an organic solvent, is coated on one surface of a carbon non-woven fabric as a gas diffusion layer by using a process similar to that for the catalyst layer, and is then fired at a temperature of about 300° C. to 400° C., thereby to burn off the surfactant to obtain a water repellent carbon layer. The thus obtained water repellent carbon layer is arranged to be in contact with the catalyst layer. According to this process, a surfactant is indispensable for making an ink from a water repellent carbon material containing e.g. water repellent fine fluorocarbon resin particles. Further, a firing process is indispensable because the surfactant in the formed structure needs to be removed.
The method of using an organic solvent for making an ink of an electrode catalyst, as above described, has problems, e.g., in its safety against ignition, environmental conservation and high cost of the organic solvents per se. Other problems are that the manufacturing process becomes complicated, the manufacturing time increases, and the cost of manufacturing apparatus increases, because a step of drying the organic solvent and a step of collecting the evaporated organic solvent are needed.
The method of using a surfactant for making an ink from a water repellent carbon material needs a firing step, so that it has the problems of a more complicated manufacturing process, longer manufacturing time and higher cost of manufacturing apparatus. Furthermore, because of the problem that the environment is polluted by the odor of incomplete combustion generated during firing, a disposal step is also needed therefor. This has been a factor which decreases productivity.
In order to put fuel cells to practical use, a further improvement of efficiency is needed. For this purpose, it is important to optimize the structure of the catalyst particles of the catalyst layer and the carbon particles carrying them. A manufacturing method for realizing these structures is also necessary.